In recent years, electronic devices such as AV equipment and personal computers are becoming portable and wireless at a rapid pace. As a power source for them, there is a growing demand for a compact and lightweight secondary battery having high energy density. In particular, a lithium ion secondary battery using lithium as an active material is greatly expected as a battery showing high voltage and high energy density. Further, a secondary battery for power tools is required to be high-powered.
To meet the above-mentioned demands, it is important to reduce an internal resistance of a battery.
In a common secondary battery, electric current is collected from a positive electrode and a negative electrode via leads, respectively. Regarding a high-powered lithium ion secondary battery, on the other hand, there has been proposed a method of collecting the electric current by protruding an electrode core of either of the electrodes from an end of an electrode group and connecting the tip of the protrusion to a current collector plate (Japanese Laid-Open Patent Publication No. 2000-294222). In this case, the current collector plate is provided with a connector piece for connecting itself to the bottom of a battery case. According to this current collecting method, the resistance can be reduced as compared with use of the leads.
In general, welding of the lead or the connector piece to the battery case is carried out by making use of a hollow part defined as a winding core of the electrode group. For example, there has been a method of spot-welding them by using a welding rod or irradiating a laser beam or an electron beam.
If there is a hollow in the winding core of the electrode group as in a cylindrical battery, it is easy to weld the current collector plate and the bottom of the battery case. However, if there is no hollow in the winding core of the electrode group as in a prismatic battery, the above-mentioned current collecting method cannot be adopted. Further, as long as the method of resistance-welding the current collector plate and the bottom of the battery case by inserting the welding rod into the winding core is adopted, improvement in capacity based on the reduction of the hollow in the winding core is not expected. Moreover, entanglement of the separator occurs when the welding rod is inserted, which may cause failure.
On the other hand, there has also been proposed a spot-welding method by irradiating a laser beam or an electron beam from outside the battery case (Japanese Patent Publication No. 2937456). However, in the case of performing laser welding from outside the battery case because a gap is prone to be formed between the current collector plate and the battery case, the welding strength is apt to vary. To eliminate the gap, pressure needs to be applied from above to the electrode group. However, this may cause failure such as buckling of the electrode plates.